The reports from Israel that it is seeking 5,000-pound GBU-28 bunker-busters for possible use against Iran makes for exciting reading. One problem: they can’t even dent Iran’s most protected sites. The reports suggest war is possible, without noting that these bombs would be like using a pea-shooter against your brother when he’s inside Dad’s car with the windows rolled up.

Assorted defense types say the only non-nuclear bomb in existence with any chance of doing real damage to the Iranian sites is the 15-ton GBU-57. Unfortunately for Israel – and perhaps fortunately for Iran – the only plane built to carry this behemoth is the B-2 bomber, which is only flown by the U.S. Air Force. Perhaps that’s why Defense Secretary Leon Panetta told Yochi Dreazen of National Journal on Thursday that an Israeli attack “would have an impact, but I think it’s also clear that if the United States did it we would have a hell of a bigger impact.”

Here’s some background on how the GBU-57 came to be:

Three years ago, the Air Force quietly circulated a solicitation seeking a “Quick Reaction Capability” to “defeat a specific set of Hard and/or Deeply Buried Targets.” The weapon, the service said, would “maximize effects against Hard and/or Deeply Buried Targets (HDBTs), while minimizing time over target.” The Air Force said it needed the weapon to meet “Urgent Operational Needs requirements” – generally a plea from a battlefield commander who doesn’t think he has the weapons he needs to accomplish a mission assigned to him.

“The system will hold at risk those highest priority assets essential to the enemy’s war-fighting ability, which are heavily defended and protected,” the Air Force said in February 2011 budget documents, “providing a critical global strike capability not currently met by inventory conventional weapons.”

The $15 million Massive Ordnance Penetrator weighs in at nearly 30,000 pounds, six times the heft of the existing GBU-28 bunker busters. Guided to its destination by GPS-guided lattice-type fins, its alloy steel hull – some 80% of its weight – is designed to remain intact as it drills through rock or reinforced concrete before setting off its 5,300-pound warhead. Air Force officials say it represents a “bridge” capability between existing bunker busters and nuclear weapons themselves.

Nearly three years ago, the Air Force said it hoped its MOPs could “reach Required Assets Available by June 2012.” But just as the Iranians have pressed their nuclear ambitions, the Pentagon’s war planners accelerated their efforts, too. The Air Force has let it been known that there’s now an operational stockpile of the world’s most powerful non-nuclear bunker busters at Whiteman Air Force Base, Missouri. They’re not far from the B-2 bombers now ready to carry them 7,000 miles to Iran’s rock-sheltered Fordow nuclear plant. But those are the only planes that can get them there.